1. Field of the Invention
The present invention relates to an image forming device in which a toner image formed on the surface of an image carrier such as a photoconductive member is transferred onto a transfer medium such as a transfer sheet, and then any residual toner after transfer adhering to the surface of the image carrier is scraped off and removed by a cleaning blade. Also, the present invention relates to a process unit used in the image forming device and an image forming method.
2. Description of the Related Art
In recent years it has become difficult to properly clean the residual toner after transfer adhering to the surface of an image carrier such as a photoconductive member using a cleaning blade in this type of image forming device. This is because the toner used in forming images is mainly made by the polymerization method instead of by the pulverization method. Specifically, toner made by the long used pulverization method had large particle diameter, with the average particle diameter in the range ten to several tens of microns, and the average circularity of toner particles was less than 0.9, or irregular shaped. With this type of toner it has become difficult to achieve the high level of dot reproducibility corresponding to the high image quality of recent years.
Therefore in recent years toner made by the polymerization method is mainly used instead of toner made by the pulverization method. The average particle diameter of toner made by the polymerization method is small, at 9 microns or smaller, also the average circularity of toner particles is 0.96 or higher, or almost a true spherical shape. By using this type of toner it is possible to achieve the level of dot reproducibility corresponding to the high image quality of recent years. However, because of the small diameter and spherical shape toner made by the polymerization method can easily roll on the surface of the image carrier and easily pass through the part where the image carrier contacts the cleaning blade. Because of the occurrence of this passing between the image carrier and cleaning blade it is difficult to scrape off the toner by the cleaning blade.
Therefore, in recent years there has been a trend to make the cleaning blade contact the image carrier with a reasonably strong force, so that passing through of spherical toner made by the polymerization method is reduced, and the cleaning performance is increased.
On the other hand, in Japanese Patent Application Laid-open No. 2002-82468 a test is described in which printing is carried out using a photoconductive member having an extremely small coefficient of friction of 0.002 as the image carrier, and with the cleaning blade pressed against the photoconductive member with the extremely high line pressure of 23.0 g/cm. In this test good cleaning of irregular shaped toner made by the pulverization method was possible throughout a long printing period of 25,000 sheets. The reason a photoconductive member with a low coefficient of friction of 0.002 was used was to reduce turning over or wear of the cleaning blade which pressed against the photoconductive member with the extremely high line force of 23.0 g/cm.
However, in this test irregular toner made by the pulverization method was used. In the case of irregular shaped toner which is easy to clean, even if the cleaning blade does not contact with the very high line pressure of 23.0 [g/cm] it is possible to remove the toner sufficiently from the photoconductive member. Specifically, in order that the toner can be cleaned well from the photoconductive member using a cleaning blade (hereafter referred to simply as a blade), it is necessary that the blade in contact with the photoconductive member properly traps the toner on the photoconductive member. In the case of small diameter spherical shaped toner made by the polymerization method, there is a gap between the photoconductive member and the blade in contact with the photoconductive member, through which the spherical shaped toner can pass. Also, the spherical shaped toner can easily roll in this gap, until it eventually passes between the area of contact of the photoconductive member and the blade. Therefore, when using spherical shaped toner, the blade is pressed against the photoconductive member with extremely high pressure, to make the gaps formed between the photoconductive member and the blade as small as possible. However, in the case of large diameter irregular shaped toner made by the pulverization method, even if there is a slight gap between the photoconductive member and the blade in contact with it, the blade can trap the irregular shaped toner well. It depends on the material of the blade, but if the line pressure of the blade is set to about 10 [g/cm], it is possible to clean irregular shaped toner sufficiently. Therefore, in this test it can be said that the irregular toner made by the pulverization method was cleaned from the photoconductive member with the cleaning blade contacting the photoconductive member with an excessively high line pressure.
In the case of using spherical shaped toner made by the polymerization method, the blade line pressure of 23.0 [g/cm] in this test is not such a high value. However, in this test, to prevent turning over of the blade that contacts the photoconductive member with this line pressure, a photoconductive member with an extremely small surface coefficient of friction was used (surface coefficient of friction=0.002). The two inventors discovered that using this type of photoconductive member it was not possible to clean spherical shaped toner well, for a reason that is explained as follows.
In other words, in order to properly clean the toner with a blade, in addition to properly trapping the toner on the photoconductive member with the blade, it is necessary that the toner that is accumulated successively on the surface of the blade drops from the surface of the blade. This is because if the accumulated toner does not drop from the surface of the blade the cleaning is not complete. The main factor to make the accumulated toner drop from the surface of the blade is vibrations of the blade. Therefore it is desirable that the blade vibrates vigorously, to make the accumulated toner drop from the surface of the blade. However, in the case of irregular toner made by the pulverization method, due to the irregular shape comparatively large gaps are formed between the individual particles of toner, so the accumulated toner on the blade is in a state that is easily broken down. Therefore, even if the blade does not vibrate so much, the toner accumulated on the blade can easily fall off the surface of the blade.
However, in the case of spherical toner made by the polymerization method, it is difficult to form gaps between the individual particles of toner, so the toner accumulated on the blade is in a state that is difficult to break down. Therefore, if the blade does not vibrate vigorously the accumulated toner cannot be effectively forced to drop off the surface of the blade, so the toner accumulated on the surface of the blade increases to a certain amount. Also, the increased amount of accumulated toner presses the blade upwards, and passes between the blade and the photoconductive member. In this test, a special photoconductive member with an extremely low surface coefficient of friction (0.002) was used, so it is not possible to make the blade that is in contact with the photoconductive member vibrate well. Therefore, when using spherical toner made by the polymerization method the toner accumulated on the surface of the blade grows to a certain size, which causes the accumulated toner to pass through the blade and the photoconductive member.
Technologies relating to the present invention are also disclosed in, for example, Japanese Patent Application Laid-open No. 2001-350287 and Japanese Patent Application Laid-open No. 2005-215242.